1. Field of the Invention
The present invention relates to interferometry. More precisely, the present invention relates to methods and apparatus for a vibration-insensitive point-diffraction interferometer. The methods and apparatus of the present invention may be implemented in measuring systems that measure various parameters of test objects by effectively removing the effect of vibration.
2. Description of the Related Art
Interferometry is a well-established method of measuring various parameters of test objects. Interferometry requires generating two waves; one is generally named the reference wave and the other the measurement wave. The reference wave is generally formed by either a plane wave using the beam of light reflected from a flat surface or a spherical wave using the beam of light generated by means of point diffraction using a pinhole or optical fiber. The measurement wave is generated by either transmitting light through test objects or having light reflected from test objects.
A practical problem encountered in performing interferometry is the presence of vibration, which causes unwanted fluctuation in interference fringes obtained between the reference and measurement waves. The vibration effect is generally considered the main cause of deteriorating measurement accuracy. In order to cope with the vibration problem, several vibration-desensitization methods have been proposed as results of previous work. A method is adopting an active means of monitoring the fluctuation of interference fringes due to vibration using a sensor with subsequent fast moving the reference surface, thus stabilizing the interference fringe (see T. Yoshino et al., Opt. Lett., 23, p. 1576). Another method is using a spatial phase-shift method to capture interference fringes in a very short period time, minimizing vibration fluctuation (see R. A. Smythe et al., Opt. Eng., 23, p. 361). Another method is using a diffraction grating in combination with a pinhole to obtain three spatially phase-shifted interference fringes at the same time (see Osuk Y. Kwon et al., Opt. Lett., 12, p. 855). Another method is using a wave-splitting element that splits the interference wavefront into a plurality of sub-waves with different phase shifts (see U.S. Pat. No. 6,304,330).